Isotopically selective Doppler-free three-photon excitation of atoms

A system for three-photon Doppler-free excitation of two-level atoms is proposed. It is shown that this is equivalent to the excitation of a two-level system with an effective interaction Hamiltonian. An analysis is made of a system for selecting the detuning of the excitation frequencies that can completely compensate for Doppler broadening. Numerical estimates are made and these show that even for small isotope shifts and high temperatures of the excited gas, excitation having a high probability and a high isotopic selectivity can be achieved.